1. Field of the Invention
The present invention relates to engine starter gearing, and more particularly to a starter drive of the type in which the gearing is automatically engaged and maintained in mesh until the engine has attained a sufficient rotary speed to indicate that it is reliably self-operative and further of the type in which the gearing is automatically maintained out of mesh after the engine has been started.
2. Description of the Prior Art
In starter gearing of this character, it is necessary to provide some form of overrunning connection between the drive pinion and the starting motor in order to prevent acceleration of the starting motor to excessive speeds when the engine becomes self-operative, and also to avoid having the starter motor act as a load on the engine during the initial period when the engine cylinders start firing. Various types of overrunning clutches have been and are employed for this purpose, but it has been found that the type which employs clutch members having interengaging teeth which are wedged together by screw jack action under load is in many respects the most satisfactory. The screw jack action is provided by a hollow screw shaft with a three (3) start helical splines machined therein. This hollow screw shaft is mounted on a driving sleeve which is in turn connected to the drive shaft.
In prior art devices, the overrunning function is provided by milling a notch in the center helical thread of the screw shaft and milling a notch so that a detent may engage this notch and stop the control nut from being driven backwards (away from the engine ring gear) after the engine has been started. The assembly of the control nut to the screw shaft is simplified by the fact that there are only three helical threads which provide the screw jack action. In assembly, the detent is merely lined up with the middle helical thread containing the notch so as to insure that the starter drive will function properly and allow the detent to engage this notch during the specific mode of operation mentioned above.
The machining of the helical spline on the hollow screw shaft and the detent notch adds a great deal of expense to the total cost of the shaft. To reduce this expense, it was determined to change the screw shaft helical spline to a 15 start spline which could then be manufactured by rolling the thread onto the screw shaft sleeve at the same time that the sleeve was being manufactured on a screw machine. This reduced the cost of the sleeve as well as the overall cost of the starter drive. However, the 15 start helical thread presented a first difficult problem for the location and milling of the detent notch. In addition, a second difficult assembly problem, was also presented in that, the assembler must align the detent of the control nut with the helical thread, the detent notch is located on, in order to insure proper functioning of the starter drive. Unless the detent is started on the same thread having the detent notch, the control nut will not properly engage the detent notch and, therefore, the starter will not function according to its intended use. In addition, the determination of whether the detent in the control nut had been properly started on the correct thread could not be made until after the sub-assembly was complete. This resulted in a considerable number of sub-assemblies being rejected and torn down for reassembly because the detent in the control nut was not properly aligned with the detent notch on the 15 start helical spline of the screw shaft.